Imaging systems which produce images of the interior of an examination object are nowadays a standard feature of modern medicine. Ultrasound systems, magnetic resonance systems, X-ray equipment or X-ray based computed tomography (CT) systems are typically used for this purpose. To facilitate identification of particular structures inside an examination object, contrast media are commonly used and image data of the region of interest (ROI) is produced at one or more points in time as the contrast medium spreads through the examination object. The contrast medium, which spreads, accumulates and dissipates again in a particular manner in or on the particular structures, makes these structures better distinguishable in the images and shows up pathologies or dysfunctions. In addition, particular organs such as blood vessels, for example, or other objects can be more easily segmented in the image data.
For many areas or rather evaluations, precise determination or even control of the accumulation state (often also termed “enhancement”) of the contrast medium in a particular object or organ is advantageous or even necessary. This applies particularly to CT angiography, for example, in which the enhancement in the vessels must be defined as accurately as possible. That is to say, on the one hand a certain minimum enhancement shall be achieved so that segmentation of the vessels in the image data can be optimally performed for subsequent evaluation. On the other hand, the enhancement must not be excessively high, in order to ensure that e.g. calcium deposits are not obscured. Also, acquisition of the image data (the CT scan) must be started at the correct point in time so that the tissue is scanned where possible at the time of maximum accumulation/enhancement. In order to achieve this, a reliable prediction of the contrast medium behavior for each individual patient is desirable.
In order to be able to predict the enhancement at a particular point in time as accurately as possible, a so-called test bolus is often injected. The patient or test subject is first given a small amount of contrast medium and then the enhancement in a particular slice, preferably in the region of interest or at least close to this region, is evaluated. In the case of CT scans, only a slice approximately 10 mm thick, generally orthogonal to the body axis, is measured, usually with a low X-ray dose. For CT angiographies this measurement usually takes place such that the enhancement is observed in a transverse section through the aorta at the heart. If other organs are to be examined, the measurement is accordingly carried out preferably in a slice in the vicinity of the respective organ, e.g. a transverse scan in the aorta near the liver in the case of a liver examination. The relevant aorta transverse section is then used as the ROI for the evaluation and the accumulation of the contrast medium as a function of time is observed using the image data in this region. It is then attempted to distill a patient-specific contrast medium impulse response function (usually also termed “patient function”) from the thus determined patient-specific test bolus contrast medium behavior data. Using this patient function, the contrast medium behavior for a further contrast medium injection can then be predicted or, conversely, if a particular contrast medium behavior is desired, the injection protocol required for that purpose can be determined. Injection protocol is to be understood as meaning control rules as to how much contrast medium is to be administered at which point in time. The function which describes this contrast medium injection as a function of time is also generally termed “input function”.
If, for example, the blood circulation system is considered as a linear time-invariant system, a contrast medium behavior can be described mathematically as a convolution of the input function with the patient function. In order to determine the patient function from the test bolus contrast medium behavior data for a known test bolus input function, a deconvolution in the spectral domain can be carried out, for example, after Fourier transformation of the injection protocol and test bolus contrast medium behavior data. However, this assumes that the test bolus contrast medium behavior data is measured over a sufficiently long period of time and with high temporal resolution so that, for example, the recirculation of the contrast medium is also included in this data. If the temporal resolution is too low or the test bolus contrast medium behavior data is not completely measured for a sufficiently long time, it is very difficult to generate a reliable patient function.